Observed ozone over the Chesapeake Bay land-water interface: The Hart-Miller Island Pilot Project.

Enhanced ozone concentrations at land-water interfaces create National Ambient Air Quality Standard (NAAQS) compliance issues across the United States. The northern Chesapeake Bay experiences higher ozone at sites adjacent to the Bay, creating ozone compliance concerns for the state of Maryland. Accordingly, the Maryland Department of the Environment sited an ozone monitor at Hart-Miller Island (HMI) within the northern Chesapeake Bay (NCB) and gathered a continuous ozone and meteorological record over 278 days within the 2016 and 2017 ozone seasons. The representative water site was the highest ozone monitor in the state 28% of all days and 75% when any ozone monitor in the state experienced ozone above the 2015 ozone NAAQS (70 ppbv), known as an exceedance day. In total, 24 exceedance days were observed at HMI. Numerical ozone predictions produced by an operational version of the Community Multi-scale Air Quality (CMAQ) model forecast 52 such days with a high bias of 15.5% in daily maximum ozone concentration during the same period. Trajectory modeling indicated over 70% of exceedance days possessed northwesterly transport over the Baltimore area, with HYSPLIT trajectories descending at least 500 m in greater than 80% of cases toward the NCB surface. These trajectories possessed a button-hook pattern during descent to create southerly surface winds at HMI that may impact coastal sites, creating ozone events at Maryland monitors such as Edgewood. Consequently, the NCB was influenced by the residual layer and from both regional long-range transport and locally sourced ozone precursors. Changes in local meteorology and emissions had a significant impact on over-water ozone concentrations and forecasts. Results of the multi-season ozone pilot study over the Chesapeake Bay provided a conceptual model of high ozone development over water downwind of a large urban center and guidance for future study of the NCB area. : Multi-seasonal observations of surface ozone and meteorology over the water of the northern Chesapeake Bay showed specific conditions leading to degraded air quality. The novel data set collected offers a deeper understanding of over-water ozone magnitude, occurrence, and transport across the land-water interface and comparison to air quality models not before possible.